White paper: A vision for a computing initiative for MFE. Revised version

The scientific base of magnetic fusion research comprises three capabilities: experimental research, theoretical understanding and computational modeling, with modeling providing the necessary link between the other two. The US now faces a budget climate that will preclude the construction of major new MFE facilities and limit MFE experimental operations. The situation is rather analogous to the one experienced by the DOE Defense Programs (DP), in which continued viability of the nuclear stockpile must be ensured despite the prohibition of underground experimental tests. DP is meeting this challenge, in part, by launching the Accelerated Strategic Computing Initiative (ASCI) to bring advanced algorithms and new hardware to bear on the problems of science-based stockpile stewardship (SBSS). ASCI has as its goal the establishment of a ``virtual testing`` capability, and it is expected to drive scientific software and hardware development through the next decade. The authors argue that a similar effort is warranted for the MFE program, that is, an initiative aimed at developing a comprehensive simulation capability for MFE, with the goal of enabling ``virtual experiments.`` It would play a role for MFE analogous to that played by present-day and future (ASCI) codes for nuclear weapons design and by LASNEX for ICF, and provide a powerful augmentation to constrained experimental programs. Developing a comprehensive simulation capability could provide an organizing theme for a restructured science-based MFE program. The code would become a central vehicle for integrating the accumulating science base. In the context the authors propose, the relationship would ultimately be reversed: computer simulation would become a primary vehicle for exploration, with experiments providing the necessary confirmatory evidence (or guidance for code improvements)